This invention relates to food and more particularly to a thermoplastic food formed by an extrusion apparatus having a steam injection means and mixing means combined therewith to provide a process for producing a food, the process, and the resultant apparatus.
Within the food processing art, a common piece of machinery which achieves a number of purposes is an extruder. The extruder is basically a tubular device having one or more rotatable screws mounted therein. At one end of the tube, is an inlet for admitting a dough to be processed into a food. The inlet communicates with a feed section of the screw. The screw while rotating within the tube pushes the dough from the feed section through the tube to the transition section of the screw and then to the metering section of the extruder screw. In the transition section of the extruder screw, the components of the dough may be mixed, or cooked, or cooled, or have a desired temperature imparted thereto. The dough may also be cooked further and have its structure altered by high shear as it advances to the metering section. Any number of desired properties can be imparted to a dough having an appropriate composition with proper processing within the extruder. After the metering section, the dough is ready to leave the extruder for further processing.
There are a number of important parameters, such as extruder screw design, revolutions per minute of the screw, back pressure, barrel temperature, die size, output temperature, external temperature of the extruder, dough viscosity, and other parameters, all of which can have a substantial effect on the output of an extruder. For example, to achieve a high, efficient throughput through the extruder, the viscosity of the dough at the inlet needs to be greater than the viscosity of the extrudate at the outlet, thus helping to push the processed dough through the extruder.
As above-mentioned, it is desired to have the dough at the feed inlet at a viscosity sufficiently high to aid in the pumping of the dough through the extruder. This viscosity adjustment is very critical in that it must be high enough to aid in pumping, yet not too high to require a substantial amount of power in this energy conscious age. Generally speaking extrudable doughs have a viscosity in the range of 10 to 200 poise. This viscosity difference between the inlet and outlet of the extruder provides for efficient pumping by the extruder screw through the extruder. As above stated while the dough is being transferred through the extruder it may be mixed, heated, or otherwise treated. Heat applied to the dough while the dough is in the extruder permits gelatinization of starch, denaturation of protein, reduction of microbiological counts and other physical and chemical changes to take place in the dough. Some heat is provided by shear or the friction of the screw turning against the dough (especially in the metering section). Other heat may be applied by a source other than the friction. For example, jackets can be placed around the tube and can be supplied with a heated fluid to indirectly heat the food. The tube may also be heated by electricity, with steam, or in any other suitable fashion.
Usually, the viscosity of a thermoplastic dough decreases with increasing temperature. This viscosity reduction is desirable when the dough is near the outlet of the extruder, because it is not desired to create high back pressures, (by back pressure is meant a pressure which tends to hold the dough in the extruder) and keep the dough within the extruder. It is desired to minimize the back pressure which can reduce the pressure keeping the dough in the extruder and thereby allow the dough to exude from the extruder as an extrudate. Thus, it is clearly desirable to have a higher viscosity for the dough at the extruder inlet than at the extruder outlet. The viscosity at the extruder outlet should be low relative to the viscosity of the food at the extruder inlet.
These aspects of extruder technology are discussed in more detail by Harmann et al Journal of Food Science, "Modeling a Forming Foods Extruder" pages 1099-1104, Volume 39, (1974), and Rossen et al Food Technology, "Food Extrusion" pages 46, 48-53, August 1973, both articles being incorporated herein by reference.
There are many ways to achieve the desired high viscosity of a dough prior to extrusion--one way being to change the formulation. However, it is also desired that the overall process be efficient and that the final product be acceptable and economical to produce. A change to a high viscosity formulation is not always the best means of achieving overall process efficiency. Another means of achieving a viscous dough at the inlet is to reduce the moisture content of the dough. However, this reduction in moisture at this level must be done while keeping in mind the moisture content required for the final product. Moisture content of the product is a major contributor to palatability, taste, and texture. Moisture content of the product also has a substantial effect on appearance. Thus, the moisture content of the dough cannot be reduced to such a point that it will have an adverse effect on the composition, appearance, or cost of the final product. Yet a dough suitable for extrusion must have a moisture content low enough for the dough to be viscous.
Also, it is desired to heat and pasteurize or process products to a certain temperature during the course of the extrusion. This heating may be started by preheating the dough before it enters the extruder. But such preheating reduces the viscosity and thus makes it more difficult to transfer it through the extruder quickly and efficiently. This problem is especially troublesome with thermoplastic food doughs. The inherent nature of any thermoplastic material (including food) is that it softens or becomes less viscous when heated. Thus if the desired temperature is reached in part by increasing the dough temperature at the inlet the required high viscosity is not maintained.
In view of the extrusion process which results in a substantial heating of the extrudate, moisture again becomes a problem. A portion of the moisture content originally present in the dough as it enters the feed section can be lost as the extrudate exits from the extruder. As is set forth above, the metering section applies both heat and pressure (from shear and other sources) to the extrudate preventing the loss of moisture from the extrudate while the extrudate remains in the extruder. As the extrudate exits from the extruder, however, pressure is released, and the extrudate is exposed to lower temperatures. Basically, the temperature outside of the extruder is less than the temperature inside. This reduction of temperature and reduction of pressure results in moisture flash-off. Thus, the extrudate almost inherently has a lower moisture content than the original dough, because of this moisture flash-off.
An extruder and its backup equipment are capital intensive and energy intensive. To increase the output and efficiency of each extruder is therefore extremely desirable. It is thus obvious that it is extremely desirable to increase extruder output while maintaining final product quality.